1. Field of Invention
The present invention relates to the field of marine seismic data acquisition systems and methods of using same. More specifically, the invention relates to automatic systems and methods for positioning of a marine seismic source, or other point between a streamer front end and the source, using an automatic controller to steer the towing vessel.
2. Related Art
The performance of a marine seismic acquisition survey typically involves one or more vessels towing at least one seismic streamer through a body of water believed to overlie one or more hydrocarbon-bearing formations. In order to perform a 3-D marine seismic acquisition survey, an array of marine seismic streamers, each typically several thousand meters long and containing a large number of hydrophones and associated electronic equipment distributed along its length, is towed at about 5 knots behind a seismic survey vessel. The vessel also tows one or more seismic sources suitable for use in water, typically air guns. Acoustic signals, or “shots,” produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones, or receivers, carried in the streamers, digitized, and then transmitted to the seismic survey vessel where the digitized signals are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed. Often two or more sets of seismic data signals are obtained from the same subsurface area. These sets of seismic data signals may be obtained, for instance, by conducting two or more seismic surveys over the same subsurface area at different times, typically with time lapses between the seismic surveys varying between a few months and a few years. In some cases, the seismic data signals will be acquired to monitor changes in subsurface reservoirs caused by the production of hydrocarbons. The acquisition and processing of time-lapsed three dimensional seismic data signals over a particular subsurface area (commonly referred to in the industry as “4-D” seismic data) has emerged in the last decade or so as an important new seismic prospecting methodology. When conducting repeated surveys, ideally one wants to repeat all source and receiver positions from the base or previous survey. In practice, this is hard to achieve for the entire survey area due to the different environmental conditions encountered in different surveys. Varying currents, both spatially and in time, are the main environmental contributor.
When conducting surveys today, a reference point at the vessel is steered automatically to be at a certain cross line distance from a given pre-plot track. A PID controller, such as that known under the trade designation Robtrack/STS500, may be used for this, and it controls the autopilot mechanism to achieve its goal. The operator sets manually how far the vessel is to be cross-line from the pre-plot line. However, this gives only control of the vessel position, and the sources and streamers will be set off to one side or the other, depending on the environmental effects in the area. When conducting time lapse surveys, the operator has to watch the source and streamer front end positions together with measurements of currents and wind to get an indication of how much the vessel should be moved to the side to achieve the steering objective. Especially when subject to rapidly varying currents, this manual steering is a real challenge, and it is often difficult to position the sources and streamer front ends to be within the specifications all the time. Manual interaction like this is also a slow process with a low update rate that is prone to inducing oscillations. Small deviations may not be reacted upon which might lead to too slow reaction when the current situation is changing. How good the steering will be also depends on the operator, meaning his skill level and level of alertness.
While there have been some efforts to use information regarding environmental conditions, including ocean currents, in vessel steering, previous attempts have not provided the desired precision in positioning marine seismic spread elements.